Well-developed larval Xenopus laevis (NF stages 58­66) are oxygen regulators, at least during mild hypoxia. When and how they change from oxygen conformers (the presumed condition of the fertilized egg) to oxygen regulators is unknown. Also unknown is how anaerobic metabolic capabilities change during development, especially in response to acute hypoxia, and to what extent, if any, anaerobiosis is used to supplement aerobic metabolism. Consequently, we have investigated resting rates of oxygen consumption (M.O2) and concentrations of whole-body lactate (lactic acid) during development in normoxia and in response to acute hypoxia in Xenopus laevis. M.O2 increased in an episodic, non-linear fashion during development. Resting, normoxic M.O2 increased about tenfold (to approximately 0.20 µmol g-1 h-1) between NF stages 1­39 and 40­44, and then another tenfold between NF stages 45­48 and 49­51 (to approximately 2.0 µmol g-1 h-1), remaining at about 2 µmol g-1 h-1 for the remainder of larval development. M.O2 reached its highest level in newly metamorphosed frogs (nearly 4 µmol g-1 h-1), before decreasing to about 1.0 µmol g-1 h-1 in large adults. X. laevis embryos and larvae up to NF stage 54­57 were oxygen conformers when exposed to variable levels of acute hypoxia. The only exception was NF stage 45­48 (external gills present yet body mass still very small), which showed some capability of oxygen regulation. All larvae older than stage 54­57 and adults were oxygen regulators and had the lowest values of Pcrit (the oxygen partial pressure at which M.O2 begins to decline). Whole-body lactate concentration in normoxia was about 1 µmol g-1 for all larval groups, rising to about 12 µmol g-1 in adults. Concentrations of lactic acid in NF stages 1­51 were unaffected by even severe ambient hypoxia. However, whole-body lactate levels in NF stages 52­66 increased in response to severe hypoxia, indicating that some anaerobic metabolism was being used to supplement diminishing aerobic metabolism. The largest increases in concentration of lactate occurred in late larvae and adults.
Developmental changes in oxygen consumption regulation in larvae of the South African clawed frog Xenopus laevis
